1. Field of the Invention
The present invention relates to the field of brightness enhancement for liquid crystal displays for watches and similar devices.
2. Description of the Prior Art
The operation of liquid crystal display devices has typically been based upon the physical principle that the liquid crystalline substance suffers a change of state upon application of a suitable electric potential. Upon application of a voltage across the liquid crystalline substance, which voltage is equal to or greater than the threshold voltage for the particular liquid crystalline substance and device design, the substance will change to an "excited" state. In the excited state the liquid crystalline substance has a greater optical coefficient of scattering than the "unexcited" portions of the liquid crystalline substances. The differential in the optical coefficient of scattering between the "excited and unexcited" states can then be exploited to form alphanumeric displays suitable for miniaturized optical displays, such as digital watch display. Displays displaying this principle are commonly referred to as dynamic scattered liquid crystals displays. The subject invention may also be applied to liquid crystal displays employing different principles (e.g., field effect) and passive (non-radiating) displays (e.g., electrochromic). For purposes of clarity the detailed description is directed to liquid crystal displays of the dynamic scattering type.
Since the basis of operation for a liquid crystal display is derived from the change of state of a passive optical characteristic of the liquid crystal substance, and involves no release of optical energy, it is necessary in some circumstances to employ an independent light source in order to conveniently see the change of state. The possible modes of illumination of the liquid crystalline substance are divided into two basic groups, a transmissive mode and a reflective mode. In the transmission mode the liquid crystalline layer is positioned between the light source and the observer. In the reflective mode the observer and the light source are positioned on the same side of the liquid crystalline layer. The transmissive mode is shown in U.S. Pat. No. 3,540,796, FIGS. 2 and 3. In that patent, the observer's line of sight is substantially perpendicular to the plane of the display read out. Light from the light source appears to uniformly brighten the plane of the display read out. When portions of the liquid crystalline substance are selectively "excited" those portions preferentially scatter the light to a greater degree than "unexcited" portions. The desired display appears as darkened regions in a brightly illuminated field.
Alternatively, in one type of reflective mode device, light is directed from an exterior light source through the plane of the display read out and the liquid crystalline substance. The light is then typically absorbed in a darkened back plate. In such a device the display read out appears uniformly darkened. Selective excitation of a portion of the liquid crystalline substance scatters the incident light to a greater degree. This causes those portions to appear as brightened regions in a uniformly darkened background. However, the brightness of the image in such a device depends both upon the magnitude of the applied electric field within the display device and the brightness of the incident light.
Of the two basic modes, a reflective mode device is the least efficient. Only a small fraction of the incident light is back scattered to the observer by reflection. It has been reported that only about 10% of the incident light is scattered away from the direction of incidence. Thus, only a small fraction of incident energy is made available for the read out display. (See U.S. Pat. No. 3,499,112).
In an attempt to increase the observed brightness of the reflective type display, the prior art has devised a second "type" of reflective mode. A reflective coating (rather than an absorptive coating) is placed on the side of the liquid crystalline substance opposite from the observer. Preferably, a collimated light source, (e.g., sunlight), is incident upon the plane of the read out display at an angle equal to or less than 45.degree.. Incident light penetrates the plane of the display read out; it transmitted through the liquid crystalline substance; and is reflected by an equal angle from the rear reflective layer. When portions of the liquid crystalline substance are selectively excited, a fraction of the scattered light is transmitted to the rear reflective layer through a smaller angle of incidence. The light is then reflected through that smaller angle to the observer whose line of sight is positioned substantially perpendicular to the plane of the display read out. Thus, the observer observes a brightened image on a dark background. The image is formed both by the fraction of light scattered backward toward the observer from the liquid crystal material and a fraction of the scattered light reflected from the back reflective coating. (See U.S. Pat. No. 3,675,988.)
The prior art has also devised various embodiments of the transmissive mode whereby the display read out may appear as a brightened image against a darkened background rather than a dark image against a bright background as first discussed. This effect is achieved by placing a polarizing layer between the light source and the liquid crystalline substance. A second polarizing layer is positioned between the observer and the liquid crystalline substance. The light emitted from the source is plane polarized by the first polarizing layer. It is transmitted through the liquid crystalline substance. The light is then incident upon the second polarizing sheet. By orienting the axis of polarization of the second sheet perpendicularly to the axis of polarization of the first sheet, substantially all of the transmitted light may be cross-polarized and the field of illumination may be made to appear uniformly dark. When portions of the liquid crystalline substance are selectively excited, the direction of polarization of the scattered light from the excited portions of the substance is rotated by varying degrees. It is then partially transmitted through the second cross-polarizing sheet. The light incident upon the observer then appears as a brightened image upon a darkened background. (See U.S. Pat. No. 3,711,713.) Alternatively, by orienting the optical axis of the second polarizing layer substantially parallel to the optical axis of the first polarizing layer, plane polarized light transmitted to the first polarizing layer travels through the liquid crystalline substance and the second polarizing layer with little attenuation. Accordingly, a brightly illuminated field is created. However, when portions of the liquid crystalline substance are selectively excited, the axis of the polarization of the scattered light is rotated out of the axis defined by the first polarizing layer. That portion of light which has its polarization axis rotated is substantially attenuated by the second polarizing layer. Thus, dark images are formed on a brightly illuminated background.
As further embodiments of the transmissive mode, the prior art has also used side illumination of the liquid crystalline substance. Light is transmitted along the liquid crystalline layer parallel to the plane of the display read out. An observer with a line of sight substantially perpendicular to the plane of the display read out observes a darkened background. When portions of the liquid crystalline material are selectively excited, a fraction of the light is side scattered. The images thus appear as bright images against a darkened background.
In each of the prior art embodiments discussed above, the display is designed to be illuminated either by a light source operating solely in the transmissive or reflective mode. It is possible in a device designed to operate in the transmissive mode, that an external light source could work to counteract the intended differential in image illumination by causing the display to be illuminated by reflection. For example, in devices designed to operate in the transmissive mode, where normal operation would form a dark image on a bright background, it is possible that ambient light could pass through, or be absorbed in, the liquid crystal device. A fraction of the ambient light would then be scattered by the excited regions of the liquid crystalline substance. This phenomenon would tend to form a bright image against a dark background. Thus, it would tend to wash out the images formed by the normal mode of operation of the device. What is needed than is transmissive illumination device of such configuration that reflected ambient light will tend to enhance rather than cancel the desired images.